Many of the vision tests, in particular tests which measure visual acuity, require the test objects to subtend an accurately prescribed angle on the retina (Foxell CAP et al, Measurement of Visual Acuity. Brit J. of Opthalmology 1955; 39:513-533). In the Snellen visual acuity test, for example, the visual angle of the test pattern (a letter, number etc.) is standardized to subtend 5 minutes of an arc with individual details of the object subtending 1 minute of an arc.
The visual angle is a function of 2 variables: the size of the object and its distance from the eye. Given an object of fixed size, the visual angle is inversely proportional to the distance of the object from the eye. And, vice versa, given a set distance between the object and the eye the visual angle is proportional to the size of the object. In many clinical tests the latter approach is used to achieve the required standard testing conditions. The distance at which the test pattern is viewed is stringently controlled, for example 20 feet for the Snellen chart, with the size of the test patterns is fixed accordingly. This approach is dictated by the fact that, once chosen, the size of the test patterns cannot be altered, and therfore only one specific distance fulfills the test criteria.
One drawback of this approach is that due to difficulties in maintaining the required distance, standardized conditions are not met and the test results are therefore not always reliable.
Using a computer to generate test patterns, removes the constraint of having to work with test patterns of fixed size, and therefore standardized conditions do not depend on conducting the test at a fixed distance, provided the computer is able to adjust the size of the test patterns according to the distance in order to meet the test criteria, or alternatively to make appropriate corrections when evaluating the results of the tests.